Right atrial pressure and venous return during cardiopulmonary bypass

Abstract

The relevance of right atrial pressure (RAP) as the backpressure for venous return (Q) and mean systemic filling pressure as upstream pressure is controversial during dynamic changes of circulation. To examine the immediate response of Q (sum of caval vein flows) to changes in RAP and pump function, we used a closed-chest, central cannulation, heart bypass porcine preparation ( = 10) with venoarterial extracorporeal membrane oxygenation. Mean systemic filling pressure was determined by clamping extracorporeal membrane oxygenation tubing with open or closed arteriovenous shunt at euvolemia, volume expansion (9.75 ml/kg hydroxyethyl starch), and hypovolemia (bleeding 19.5 ml/kg after volume expansion). The responses of RAP and Q were studied using variable pump speed at constant airway pressure (P) and constant pump speed at variable P Within each volume state, the immediate changes in Q and RAP could be described with a single linear regression, regardless of whether RAP was altered by pump speed or P ( = 0.586-0.984). RAP was inversely proportional to pump speed from zero to maximum flow ( = 0.859-0.999). Changing P caused immediate, transient, directionally opposite changes in RAP and Q (RAP: ≤ 0.002 and Q: ≤ 0.001), where the initial response was proportional to the change in Q driving pressure. Changes in P generated volume shifts into and out of the right atrium, but their effect on upstream pressure was negligible. Our findings support the concept that RAP acts as backpressure to Q and that Guyton's model of circulatory equilibrium qualitatively predicts the dynamic response from changing RAP. Venous return responds immediately to changes in right atrial pressure. Concomitant volume shifts within the systemic circulation due to an imbalance between cardiac output and venous return have negligible effects on mean systemic filling pressure. Guyton's model of circulatory equilibrium can qualitatively predict the resulting changes in dynamic conditions with right atrial pressure as backpressure to venous return.

Effect of volume status on the estimation of mean systemic filling pressure

“…The volume loaded in upstream veins as a consequence of increasing RAP during inspiration will be released into the RA as vessels recoil during expiration (9,22).…”

“…Temporary imbalances between atrial and venous in-and outflow may lead to underestimation of steady state VR (9). A new steady state MSFP is reached only after transient changes in blood flow and volume shifts have settled (2,22).…”

Reply to “Is the Guytonian framework justified in explaining heart lung interactions?” and “Venous return, mean systemic pressure and getting the right answer for the wrong reason”

“…Taken together, this illustrates how the heart operates within limits set by the inflow, not withstanding the fact that the resulting RAP simultaneously acts as backpressure to VR. We recently proved what is now objected by identifying the time series of events with a cross-correlation analysis between RAP and VR (8). In this experiment, which handled all criticized points in Guyton's original setup (9), we could clearly show that changes in RAP induced by ventilation preceded changes in VR.…”

Right ventricular stroke volume assessed by pulmonary artery pulse contour analysis

“…Right ventricular afterload increases may therefore be a reason for left-sided stroke volume variation, even though averaged right and left stroke volumes must equal each other [8]. Measurements upstream of the lung may not be exposed to these issues, since the right heart directly handles venous return and therefore changes in volume state [19,20,34]. Stroke volume variation may then be of particular interest.…”